Digital Subscriber Line (DSL) is a technology that enables broadband signals, providing services such as high speed internet access, video-on-demand and real time video conferencing, to be transmitted over ordinary telephone lines from the providers' centrally located DSL Access Module (DSLAM) to DSL modems located at the customer premises.
However, since the inception of DSL, a major limitation on its performance has been the presence of noise in the telephone line over which it operates. Noise combines with the wanted DSL signal and reduces the ability of the receiving modem to demodulate the incoming signal.
Shannon's law (C=W log 2(1+S/N), where C is the channel capacity in bits per second, W is the bandwidth in hertz, and S/N is the signal-to-noise ratio, shows that any increase in noise decreases the data carrying capacity of the signal. Therefore, the inverse is also true; any device or method which reduces the amount of noise in the line or in any of the metallic connectors that make up the connection between the DSLAM and modem, will improve the ability of the system to transfer data.
Noise has several sources, both external and internal to the telephone line over which the wanted signal is transmitted. Since it is normal practice to gather the individual conductor pairs together into a cable bundle, a major cause of noise is the cross coupling of the signal in one pair within the cable into other pairs in the same cable. This is the phenomenon known as crosstalk. Also, external electromagnetic noise will be coupled into all the pairs in the cable. A third source of noise is that which occurs within the customer premises and is sometimes caused by the operation of household appliances, air conditioners, etc. This noise is known as “impulse” noise.
Within the DSL community, noise has been accepted as endemic and little attempt has been made to reduce its presence in the conductor wires. In fact, much time, money and ingenuity has gone into finding ways to make DSL operate more efficiently in the presence of noise. There remains a need however for a system that provides a means whereby noise in the wire pairs may be substantially reduced, thereby improving the transmission of data, according to Shannon's law.